LOW-PRESSURE METAMORPHISM OF THE MAFIC VOLCANIC-ROCKS OF THE ROSSLANDGROUP, SOUTHEASTERN BRITISH-COLUMBIA

Mafic volcanic rocks of the Rossland Group have been metamorphosed in the subgreenschist to lower amphibolite facies. Subgreenschist-facies regional metamorphic rocks are subdivided into prehnite-pumpellyite zo ne and prehnite-epidote zone. Fluid inclusions in two subgreenschist-f acies veins yielded mean homogenization temperatures of 139 and 151 de grees C. Assuming a reasonable maximum temperature limit of 275 degree s C for the subgreenschist facies, the fluid-inclusion isochores indic ate a pressure <250 MPa for regional metamorphism in the subgreenschis t facies. This is consistent with the widespread occurrence of prehnit e-chlorite-bearing assemblages. Metamorphic grade increases sharply no rthward approaching the large plutons of the Nelson suite. The contact aureoles of the Nelson batholith and the related Bonnington pluton en compass most of the region, producing an extensive region underlain by rocks within the hornblende-oligoclase zone. Intrusion of the Nelson plutonic suite overlapped with the development of the Hall Creek syncl ine and Silver King shear zone. The pattern of isograds across the Ros sland Group indicates superimposed contact and regional metamorphism r ather than progressively deeper structural levels northward.